Known for determining process variables of a medium in a container are, among others, vibronic measuring devices. These are offered by the assignee in a large number of variants under the marks Liquiphant for liquids and Soliphant for bulk goods. Such vibronic measuring devices have an oscillatable unit in the form of an oscillatory fork, which is composed of a membrane and two paddles protruding therefrom into the medium, or in the form of an oscillatable rod. The oscillatable unit is excited to resonant oscillations, and a change in the oscillation frequency and/or the amplitude of the oscillations and/or the phase between the transmission signal and the received signal is evaluated with respect to the process variable, principally a limit-level or the density of the measured medium. In the case of a vibronic limit level switch for liquids, a distinction is made, for example, between the free state, i.e. a freely oscillating oscillatory fork, and the covered state, i.e. an evaluated oscillatory fork covered by medium. The two states have different resonance frequencies. Furthermore, with such an apparatus, also density and viscosity of the medium are determinable or a phase boundary is detectable.
Other vibronic measuring devices are known for the field of flow measurement. In such case, an oscillatable tube is inserted, as an intermediate piece, into a pipeline flowed through by medium and is excited to oscillations. Via the Coriolis effect, flow velocity is determinable. From the oscillatory behavior, however, information concerning density and viscosity of the medium can also be obtained.
Excitation for execution of mechanical oscillations most often occurs by means of a piezoelectric drive, in the case of which at least one piezoelectric element coupled with the oscillatable unit is supplied with an electrical transmission signal, which it converts to a mechanical signal. Conversely, the mechanical oscillations of the oscillatable unit can be converted into evaluatable electrical signals by means of a piezoelectric receiving unit. Often, the drive unit and receiving unit are embodied as a combined drive/receiving unit and, together with a control/evaluation unit, are arranged in a control loop. This control loop controls the transmission signal in such a manner that a predeterminable phase is present between the transmission signal and received signal.
In Offenlegungsschrift DE 102009026685 A1, an alternative method for digitally controlled excitation of the oscillatable unit to mechanical oscillations is disclosed. In contrast to the analog embodiment, in the case of this largely digital solution, a forced excitation occurs at a particular frequency. In order to find that measuring frequency, in the case of which the predetermined phase shift exists between the received signal and transmission signal, a so-called frequency sweep is performed. In a frequency sweep, the oscillatable unit is excited to oscillations within a particular frequency band in the working range of the sensor and successively with discrete frequencies lying close to one another, and the frequency corresponding to the predetermined phase shift is ascertained. DE 102009028022 A1 describes an advantageous further development of the frequency sweep, which simplifies evaluation of the received signal for finding the measuring frequency, in that the received signal phase is selectively sampled and evaluated only at certain points in time.
A problem arises in the case of said method when only a small amount of time is available for the frequency sweep. The faster the frequency band is run through at a constant number of excited frequencies, the smaller the time, which is available to the oscillatory system to adapt itself to the newly presented frequency. Superpositioning effects occur, which lead to the predetermined phase shift occurring at a frequency different from the actually sought measuring frequency, and thus to the measuring frequency not being correctly ascertainable.